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Inheritance P1
AQA A-level biology inheritance year 13
| Term | Definition |
|---|---|
| Homozygous | Two alleles are the same |
| Heterozygous | Two alleles are different |
| Homogametic | The X chromosome can make both sex phenotypes |
| Heterogametic | The Y chromosome can only make the female phenotype |
| Gregor Mendel: father of modern geneticism | A monk who bred pea plants and ran quantitative experiments by tracing various characteristics to the inheritance of plants and called differences in the same genes ‘hereditary units’ but we call them alleles |
| Pure bred plants | Homozygous plants |
| Backcrossing plants | Crossing two pure bred plants making them heterozygous |
| F1 | First Filial Generation. The first generation of offspring produced by the crossing of two parents composed of heterozygous individuals with traits that are being studied |
| Dihybrid inheritance | The simultaneous inheritance of two genes on different chromosomes leading to the expression of two traits in the offspring and occurs when both parents are heterozygous for the traits |
| How to draw a dihybrid inheritance punnet square | Identify the parental genotypes (e.g., RrYy and rryy) List the alleles for each trait (e.g., R, r, Y, y) Draw a 4x4 punnet square with the top row & left column labelled with the parent alleles In each square, combine the alleles from the row & column |
| Phenotypic ratio (and how to label punnet square) | Typical F2 ratio of 9:3:3:1 most dominant to most recessive. Most dominant would be from bottom left to top right and along top row and left column. Most recessive in bottom left corner and a different genotype from top left diagonal down to bottom right |
| Independent segregation | Alleles for different traits separate randomly and independently from one another during the formation of gametes |
| Crossing over | Homologous chromosomes exchange segments of genetic material resulting in a new combination of alleles and genetic diversity in offspring |
| Chiasma (plural: chiasmata) | The physical point of contact where 2 homologous chromatids twist around each other and break off to recombine on the other chromosome |
| Random fertilisation | The number of possible combinations of gametes is 2^n (in humans n=23), but because a random egg and sperm will fuse there are actually (2^n)^2 combinations, not even including crossing over |
| Gene | A section of DNA that codes for one characteristic |
| Allele | A variation of a gene |
| Genotype | The complete genetic makeup of an organism |
| Phenotype | Observable characteristics of an organism |
| Gene pool | A range of alleles present in a population at a specific time |
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